That's an exaggeration. Nobody is trained to read STOP signs for 16 years, a few months top. And Waymo doesn't need to coordinate a four-limbed, 20-digited, one-headed body to operate a car.
That being said, you don't really need training to understand a STOP sign by the time you are required to, its pretty damn clear, it being one of the simpler signs.
But you do get a lot of "cultural training" so to speak.